Optical transmission systems, including optical fiber communication systems, have become an attractive alternative for carrying voice and data at high speeds. In addition to the pressure to improve the performance of optical communication systems, there is also increasing pressure on each segment of the optical communication industry to reduce costs associated with building and maintaining an optical network.
One technology used in optical communication systems is wavelength division multiplexing (WDM). As is well known, WDM pertains to the transmission of multiple signals (in this case optical signals) at different wavelengths down a single waveguide, providing high-channel capacity. Typically, the optical waveguide is an optical fiber.
For purposes of illustration, according to one International Telecommunications Union (ITU) grid a wavelength band from 1530 nm to 1565 nm is divided up into a plurality of wavelength channels, each of which have a prescribed center wavelength and a prescribed channel bandwidth; and the spacing between the channels is prescribed by the ITU grid. For example, one ITU channel grid has a channel spacing requirement of 100 GHz (in this case the channel spacing is referred to as frequency spacing), which corresponds to channel center wavelength spacing of 0.8 μnm. With 100 GHz channels spacing, channel “n” would have a center frequency 100 GHz less than channel “n+1” (or channel n would have a center wavelength 0.8 nm greater than channel n+1). The chosen channel spacing may result in 40, 80, 100, or more wavelength channels across a particular passband.
In WDM systems it may be useful to employ tunable optical filters in this demultiplexing process. For example, tunable optical filters may be useful in reconfigurable optical networks to facilitate a number of operations including demultiplexing. Moreover, the drive to reduce network costs and operation costs has placed a value on flexibility that has not previously existed; and that may be provided by tunable optical filters.
Unfortunately known tunable optical filters suffer certain implementation and performance drawbacks (e.g., suitably sharp cutoff outside of the passband of the filter; suitably low polarization dependent loss; and suitably low chromatic dispersion, some or all of which tend to degrade over the tuning range of conventional tunable filters).
Because the known tunable optical filters have unacceptable drawbacks, there exists a need for optical filter elements which overcome at least the drawbacks described above.